Your idea isn't to hard actually. Depending on how big the tank is there are several ways to go about it.
There have been several people who have had great success with using Great Stuff and spraying it on the back wall of the tank or onto egg crate on the back wall. After they have the general idea of what they want they then sprinkle
aragonite sand and push pieces of rock into the great stuff just before it hardens. I happen to think that this would be a very cool idea, and have found a thread that better explains it.
http://www.nano-reef.com/forums/show...ht=Great+Stuff
Another way to do it is buy buying some agrocrete mix and using Aragonite sand and rubble as your aggregates. Making sure to mix as little water into it as necessary just spread it onto a sized and cut piece of eggcrate and let that setup and cure. Once cured then placing it into the tank and mixing up another smaller more wattery batch to fill in all the cracks. The great thing about doing this, is that it allows you to use minimal ammounts of room as wells make your own ideal shaped piece of rock.
The best part about doing this is that it will eventually cover itself with
coraline algae and become one beautiful piece of what looks like rock.
